1. Field of the Invention
The present invention relates to a device for exchanging heat with a flat product.
It is more particularly applicable to any flat product, in strip or sheet form, or even one formed from a layer of parallel wires.
It relates more particularly to the field of heat treatment of rolled products, such as a rolled metal, which pass over rollers and successively traverse heat treatment chambers. Such annealing or galvanizing lines are used continuously, for example in the manufacture of steel sheets for motor car bodywork. The steel can be raised to temperatures which can reach 600-900xc2x0 C. A rapid and uniform cooling of these products is then necessary in order to bring the temperature of the product down to a temperature below 500xc2x0 C. depending on the desired quality.
2. Description of the Related Art
According to the prior art, a heat exchange device such as described in French patent FR 2,738,577 in the name of the Applicant, makes it possible to continuously cool a rolled product passing in front of a series of blades that form ducts for the ejection of a cooling gas.
French patent FR 2,738,577 discloses a device for placing a plenum chamber under gaseous pressure. The plenum chamber includes, on a front face thereof, several blades that form a duct for the ejection of the gas towards a surface of the rolled product, the blades being superimposed upon one another along the direction of movement of the rolled product and constituting an outlet orifice for the gas extending over the width of the rolled product.
Each space separating two superimposed blades has a depth in a direction perpendicular to the surface of the rolled product and a width in the longitudinal direction of the rolled product sufficient to allow the evacuation of gases without disturbing the gas outlet of the adjacent blades.
Thus, the space provided between the blades facilitates the evacuation of the gas at the level of the surface of the rolled product and does not impede the emission of gas coming out of the orifice of the blades.
In fact, if no precaution is taken to evacuate the hot gases after their impact on the product then, as the gas flow is increased, the heat transfer coefficient, and therefore the cooling speed, ceases to grow and a xe2x80x9csaturationxe2x80x9d effect then occurs. This phenomenon is described, for example, in the article by C. Brugnera et al., Revue de Mxc3xa9tallurgie, December 1992, page 1098, FIG. 8, where it can be seen that above 500 mm of column of water (CE) of pressure at the orifice, the cooling speed no longer increases, even on increasing the pressure to 800 mm CE.
In the case of French patent FR 2,738,577, in order to totally prevent the formation of a layer of hot gas on the surface of the rolled product, the space between the blades must be dimensioned such that the return speed of the gases is less than 20 m/s, which makes it necessary, if the gas is taken off laterally on only one side, for the ratio of the sum of the half-flows of two superimposed blades (that is to say the flow of one blade) at the section of passage between two blades to be less than 20. If the product to be treated is wide and the blade is in one piece in the direction of the width of the product, and if in addition the required transfer coefficient is high, blade depths that are excessive and difficult to install must be provided.
The purpose of the present invention is to improve such a heat exchange device, and in particular to facilitate the evacuation of the gas out of the device after its impact on a flat product.
The invention thus relates to a device for exchanging heat with a flat product, moving in front of the device, that is provided with a device for placing at least one plenum chamber under gaseous pressure. The plenum chamber has, on a front face thereof, several blades forming a duct for the ejection of the gas towards a surface of the flat product, with the blades being superimposed upon one another in the direction of movement of the flat product and constituting an outlet orifice for the gas extending in the direction of the width of the flat product.
According to the invention, the heat exchanger device is characterized in that the plenum chamber has a width in the direction of the width of the flat product that allows the evacuation of the gas towards the rear on both sides of the plenum chamber.
Because of the reduced width of the gaseous pressurizing plenum chamber, the evacuation of gases after impact on the surface of the flat product can be carried out on both sides of the plenum chamber, contrary to the blades forming a gas duct on the front face of the plenum chamber.
The flow of the exiting gas is consequently directed towards the rear of the device, without impeding the emission of the gas through the orifices of the blades. Any risk of stagnation of the gas on the surface of the treated product is thus carefully avoided, just as is any current of gas parallel with the surface to be processed both in the direction of the width and in that of the movement of the product.
In a practical and advantageous manner, the width of the plenum chamber is less than the width of the outlet orifice of the gas extending in the width of the flat product.
In other words, each blade thus widens towards the flat product so that the gas, after impact on the flat product, can return towards the rear of the device, on each side of the plenum chamber.
According to a preferred characteristic of the invention, the heat exchanger device includes openings for the outlet of the gas after ejection that are situated in a plane defined by a rear face opposite the front face of the plenum chamber.
The evacuation at the rear of the plenum chamber makes it possible to avoid any movement of gas along the surface of the flat product such as is produced in the conventional devices in which the plenum chambers are continuous or disposed side by side and prevent the return of the cooling gases towards the rear. Unlike the prior devices in which the evacuation of gases is carried out on the sides of the device, the gas can be evacuated from the heat exchanger device according to the invention without generating a preferential cooling of the edges of the flat product.
According to a preferred characteristic of evacuation, the heat exchanger device embodies at least two plenum chambers disposed in the width of the flat product, the spacing between the plenum chambers being such that the evacuation of the gas between the plenum chambers is carried out at a speed of less than or equal to 20 m/s.
In this way a regular evacuation of the gas towards the rear of the device is ensured, without risk of formation of turbulence that could be prejudicial to the homogeneity of the heat exchanges.
According to an advantageous and practical characteristic of the invention, the ratio between half of the gas flow in m3/s at the outlet of two adjacent blades along the width the product and the section in m2 of the space separating the plenum chambers having the blades is less than 20, the section extending in a plane parallel with the flat product and in the direction of movement of the flat product.
According to another preferred characteristic of the invention, the ratio between the speed of the gas in a plenum chamber and the speed of the gas at the outlet of the blades integral with the plenum chamber is less than 0.2.
Because of this large difference in the speed of the gas in the plenum chamber and at the outlet of the blades, the plenum chamber forms a reservoir of gas under pressure that is virtually without circulation, ensuring a uniform speed for the ejection of gases.
According to another preferred characteristic of the invention, the device for placing the plenum chamber under gaseous pressure embodies several fans adapted to supply gas to one or more plenum chambers.
The pressure of each plenum chamber can thus the regulated independently or in sub-groups of plenum chambers, making it possible to adjust, over the width of the flat product, the cooling rate according to the desired thermal profile.
Other features and advantages of the invention will be apparent when taken in conjunction with the following description and accompanying drawings.